Imaging infrared sensors image the light from a scene in the infrared wavelength range of about 0.9 micrometers to about 12 micrometers and in some cases to even longer infrared wavelengths. By contrast, visible-light sensors operate in shorter wavelengths below about 0.9 micrometers. Infrared sensor are preferred to visible-light sensors for many applications, because the infrared sensor can often image a scene through clouds, dust, and other obstructions that obscure the visible-light scene.
In a further refinement, more-recent infrared sensors include the capability to image a scene in two different infrared wavelength bands. For example, the scene may be simultaneously imaged in the medium-wavelength infrared (MWIR) band and the long-wavelength infrared (LWIR) band. The simultaneous images in two different infrared wavelength bands may be used to determine the nature of some types of features, whose characters cannot be determined either by visible light or by imaging in a single infrared wavelength band.
Complications arise when the scene is imaged in two different infrared wavelength bands. Diffraction of the light is wavelength dependent, so that the point spread function is wavelength dependent. If MWIR and LWIR wavelengths are imaged through the same aperture, the point spread function is significantly larger for the LWIR wavelengths than for the MWIR wavelengths, with the result that the MWIR image is “sharper” than the LWIR image. This difference in the images makes it difficult to compare the two images by digital analysis techniques. On the other hand, it is known to split the input beam into beams of the two different wavelength ranges, pass the two beams through different apertures, and separately image the two beams onto different infrared detectors. While operable, this technique significantly increases the cost of the sensor system by requiring two infrared detectors, and by requiring that the cooling system be substantially larger than would otherwise be the case.
There is a need to improve dual-band infrared imaging to achieve satisfactory image quality while reducing the cost of the sensor system and the envelope size of the sensor system. The present invention fulfills this need, and further provides related advantages.